1. Field of the Invention
The present invention relates to a light receiving circuit for use in an electro-optic sampling oscilloscope which receives an optical signal polarized so as to reflect a signal to be measured and which reproduces an electric signal according to the signal to be measured on the basis of the polarization state of the optical signal.
2. Description of the Related Art
An electro-optic sampling oscilloscope (hereinafter abbreviated as an xe2x80x9cEOS oscilloscopexe2x80x9d) which optically samples a signal to be measured and reproduces the waveform of the signal has conventionally been used for observing the waveform of, for example, an internal signal of an integrated circuit.
In the EOS oscilloscope, an electro-optic crystal whose plane of polarization is changed by an electric field is connected to a portion where an internal signal to be measured (hereinafter abbreviated as a xe2x80x9ctarget signalxe2x80x9d) appears, and the target signal is reproduced on the basis of the deflected state of light reflected by the electro-optic crystal. The EOS oscilloscope comprises an electro-optic probe equipped with a built-in optical system for acquiring an optical signal whose polarization state corresponds to the target signal, and a light-receiving circuit which receives the optical signal and reproduces an electric signal corresponding to the polarization state of the optical signal.
In contrast to a conventional sampling oscilloscope using an electric probe, the EOS oscilloscope has the following characteristics:
1) Since the EOS oscilloscope does not need a ground line at the time of measuring a signal, the signal can be readily measured.
2) A metal pin provided at the tip of the electro-optic probe is electrically insulated from circuitry of the oscilloscope, and hence the waveform of a target signal can be measured without distorting the state of the target signal.
3) The EOS oscilloscope utilizes an optical pulse signal, and hence the EOS oscilloscope can measure a wide frequency range extending up to giga hertz. Because of these merits, the EOS oscilloscope receives attention.
An exemplary configuration of an electro-optic probe used in the EOS oscilloscope will now be described by reference to FIG. 2. In the drawing, reference numeral 1 designates a probe head formed from an insulator, and a metal pin 1a to be brought into contact with an area where a target signal appears is fitted into the center of the probe head 1. Reference numeral 2 designates an electro-optic element (i.e., electro-optic crystal) whose plane of polarization is changed by an electric field. A reflection film 2a is provided on the side of the electro-optic element 2 facing the metal pin 1a and remains in contact with the metal pin 1a. 
Reference numeral 4 designates a half-wave plate; 5 designates a quarter-wave plate; 6 and 8 designate polarization beam splitters; 7 designates a Faraday element; 9 designates a laser diode which emanates a laser beam in accordance with a pulse signal (control signal) output from the EOS oscilloscope main unit (not shown); and 10 designates a collimator lens which collimates a laser beam emitted from the laser diode 9 in a single direction. The electro-optic element 2, the half-wave plate 4, the quarter-wave plate 5, the polarization beam splitters 6 and 8, and the Faraday element 7 are disposed on the optical path of a laser beam A collimates by the collimator lens 10.
Reference numerals 11 designates a collective lens for collecting a laser beam separated by the polarization beam splitter 6; 13 designates a collective lens for collecting a laser beam separated by the polarization beam splitter 8; and 101 and 104 designate photodiodes constituting a light-receiving circuit to be described later. The photodiode 101 converts the laser beam collected by the collective lens 11 into an electric signal and outputs the electric signal to the EOS oscilloscope main unit, and the photodiode 104 converts the laser beam collected by the collective leans 13 into an electric signal and outputs the electric signal to the EOS oscilloscope main unit.
Reference numeral 15 designates a probe body; and 17 designates an isolator comprising the quarter-wave plate 5, the two polarization beam splitter 6 and 8, and the Faraday element 7. The isolator 17 permits transmission of the laser beam emitted from the laser diode 9 and separates orthogonal polarization component of the light reflected by the reflection film 2a. 
The exemplary configuration of the conventional light-receiving circuit used in the EOS oscilloscope will be described by reference to FIG. 3. In the drawing, reference numeral 100 designates a bias power supply; 101 and 104 designate photodiodes; 102 and 105 designate resistors; 103 and 106 designate amplifiers; 107 designates a current monitor; 108 designates an analog-to-digital-converter; 109 designates a differential amplifier comprising resistors 109A to 109D and an operational amplifier 109E; 110 designates a resistor; and 111 designates an analog-to-digital converter.
In the light-receiving circuit, the photodiode 101 is biased by the bias power supply 100, to thereby produce an electric current. The electric current is amplified by the respective amplifiers 103 and 106, and a difference between the signals output from the amplifiers 103 and 106 is amplified by the differential amplifier 109, thus producing a target signal. The value of a signal output from the differential amplifier 109 is subjected to analog-to-digital conversion by the analog-to-digital converter 111. Electric currents produced by the photodiodes 101 and 104 are monitored by the current monitor 107, and the values of the currents are subjected to analog-to-digital conversion by the analog-to-digital converter 108.
The operation of the conventional light-receiving circuit will now be described.
The laser diode 10 shown in FIG. 2 is activated by a pulse signal (control signal) and emits the pulse-like laser beam A having a sampling frequency. The laser beam A is converted into collimated light by the collimator leans 9, and the thus-collimated light travels straight ahead through the polarization beam splitter 8, the Faraday element 7, and the polarization beam splitter 6 and enters the electro-optic element 2 by way of the quarter-wave plate 5 and the half-wave plate 4.
The laser beam that has entered the electro-optic element 2 is reflected by the reflection film 2a formed on the end face of the electro-optic element 2 facing the metal pin 1a. When the metal pin 1a is brought into contact to a point of measurement, the electric field corresponding to the voltage applied to the metal pin 1a is propagated to the electro-optic element 2, thus causing a phenomenon in which the double refractive indices of the electro-optic element 2 are changed by the Pockels effect. The polarized state of the light is changed when the laser beam emitted from the laser diode 9 propagates through the electro-optic element 2. As a result, the laser beam reflected by the end face of the electro-optic element 2 includes a polarization component corresponding to the voltage of the target signal.
The laser beam reflected by the end face of the electro-optic element 2 passes again through the half-wave plate 4 and the quarter-wave plate 5. A portion of the laser beam (a polarization component corresponding to the voltage of the target signal 9 is separated by the polarization beam splitter 6. The thus-separated beam is collected by the collective lens 11, and the thus-collected beam enters the photodiode 101 constituting the light-receiving circuit. The remaining laser beam that has transmitted through the polarization beam splitter 6 is separated by the polarization beam splitter 8, and the thus-separated laser beam is collected by the collective lens 13. The laser beam then enters the photodiode 104 shown in FIG. 3 and is converted into an electric signal.
The operation of the light-receiving circuit will now be described. When the plane of polarization of the electro-optic element 2 is changed in association with a change in the voltage of the target signal, there arises a difference between the signal output from the photodiode 101 and the signal output from the photodiode 104. Upon detection of the difference, the light-receiving circuit is operated so as to output a measurement signal corresponding to the target signal.
When the photodiode 101 of the light-receiving circuit receives the laser beam emitted from the polarization beam splitter, the photodiode 101 produces an electric current corresponding to the intensity of the laser beam, and a voltage corresponding to the electric current appears at one end of the resistor 102. The thus-appeared voltage is amplified by the amplifier 103. Similarly, a voltage corresponding to the electric current produced by the photodiode 104 appears at one end of the resistor 105, and the voltage is amplified by the amplifier 106. The differential amplifier 109 outputs a measurement signal corresponding to the difference between the signal output from the amplifier 103 and the signal output from the amplifier 106.
As mentioned above, in the conventional light-receiving circuit, the signal detected by the photodiode 101 is amplified by the amplifier 103, and the signal detected by the photodiode 104 is amplified by the amplifier 106. Subsequently, the difference between the signals is amplified by the differential amplifier 109, thus detecting only a measurement signal.
The electric current monitored by the current monitor 107 is subjected to analog-to-digital conversion by the analog-to-digital converter 108. The current is used for verification or calibration of operation of the photodiodes 101 and 104 together with the value of the measurement signal converted by the analog-to-digital converter 111. The plane of polarization of the laser beam that enters the electro-optic element 2 must be matched with the crystallographic axis of the electro-optic element 2. To this end, the plane of polarization is controlled by rotation of the half-wave plate 4 and the quarter-wave plate 5.
However, in the conventional light-receiving circuit, the signal detected by the photodiode 101 is amplified by the amplifier 103, and the signal detected by the photodiode 104 is amplified by the amplifier 106. If a sampling frequency is shortened as a result of an increase in a sampling rate, a difference in transient response characteristics between the amplifiers 103 and 106 becomes manifest. As a result, in-phase signal components of the photodiodes 101 and 104 appear in the form of an error of the measurement signal output from the differential amplifier 109, thus deteriorating the signal-to-noise ratio (S/N) of the waveform of the measurement signal.
The present invention has been conceived in view of the foregoing problem in the related art, and an object of the present invention is to provide a light-receiving circuit for use in an electro-optic sampling oscilloscope which prevents in-phase signal components of the photodiodes from appearing in the form of an error of a measurement signal, which would otherwise be caused when the sampling frequency becomes shorter as a result of an increase in a sampling rate, and can accurately convert a received optical signal into an electric signal.
According to one aspect of the present invention, there is provided a light-receiving circuit for use in an electro-optic sampling oscilloscope which receives first and second optical signals resulting from polarized separation of an optical signal whose polarized state reflects the voltage of a signal to be measured, and which produces an electric signal corresponding to the relative relationship between the intensity of the first optical signal and the intensity of the second optical signal, the circuit comprising:
first and second photo-electric conversion elements which are connected in series between a first bias power supply and a second bias power supply, receive the first and second optical signals, and convert the first and second optical signals into electric signals;
an amplifier which receives an electric signal appearing in a point of connection between the first and second photo-electric conversion elements and amplifies the electric signal; and
a first detector for detecting the electric signal converted by the first photo-electric conversion element and a second detector for detecting the electric signal converted by the second photo-electric conversion element.
Preferably, the light-receiving circuit further comprises a control section for controlling the ratio of polarization between the optical signals such that the difference between the value detected by the first detector and the value detected by the second detector becomes smaller when the first and second photo-electric conversion elements receive the reference light.
Preferably, the light-receiving circuit further comprises a correction section for correcting the value of an output of the amplifier in accordance with the amount of variation in the sum of the value detected by the first detector and the value detected by the second detector.